docs: remove incorrect v6 auto prune info

[wraithgar]

As of npm@7, extraneous modules are always auto pruned

[npm-robot]

found 5 benchmarks with statistically significant performance regressions

• app-large: no-modules, no-clean:audit
• app-medium: modules-only, no-lock, no-cache

timing results

app-large	clean	lock-only	cache-only	cache-only peer-deps	modules-only	no-lock	no-cache	no-modules	no-clean	no-clean audit
npm@8	55.035 ±5.44	33.422 ±1.22	32.731 ±16.08	22.423 ±1.25	3.133 ±0.03	3.090 ±0.13	2.516 ±0.14	13.448 ±0.54	2.537 ±0.05	3.238 ±0.03
#4845	57.799 ±5.66	32.977 ±1.88	28.195 ±7.96	24.595 ±0.40	3.452 ±0.02	3.274 ±0.01	2.640 ±0.01	15.277 ±0.38	2.622 ±0.01	4.026 ±0.28

app-medium	clean	lock-only	cache-only	cache-only peer-deps	modules-only	no-lock	no-cache	no-modules	no-clean	no-clean audit
npm@8	40.882 ±1.24	25.644 ±0.92	14.515 ±0.37	15.997 ±0.41	2.841 ±0.10	2.557 ±0.03	2.371 ±0.07	9.629 ±0.36	2.465 ±0.17	3.528 ±0.22
#4845	44.931 ±2.06	28.310 ±0.74	15.626 ±0.57	15.911 ±0.13	3.336 ±0.08	3.218 ±0.08	2.762 ±0.02	10.376 ±0.24	2.717 ±0.05	3.619 ±0.04

[nlf]

the change to the config definition means docs need to be rebuilt as part of this pull request too

[wraithgar]

Weird that npm run snap didn't do this automatically anymore?

[wraithgar]

@lukekarrys Any idea what's going on here? the postsnap make -s mandocs didn't update the docs.

[nlf]

mandocs is the wrong make target i think, it should just be docs so that the markdown and html files get rebuilt too no?

[nlf]

#4848 will fix the docs build issue, after it lands we can rebase this one and re-run npm run snap and should be all set